How does cancer start?

From Cell Damage: Most cancers start due to acquired damage to normal body cells leading to mutations which lead to abnormal cell growth, increased cell lifespan, and a tendency to spread.

Cancer : Usually, a single mutation in a cell causes that cell to grow uncontrollably and possibly develop into a cancer. If you get regular screenings, such as a pap smear, mammogram, colonoscopy after 50, regular health check ups, skin surveys, oral cancer evaluarions, and the like, and be alert for signs of illness, your doctor should be able to detect any abnormality early enough to intervene.